1. Field of the Invention
This invention relates to an echo canceller operating on the principle of digital convolution.
2. Prior Art
This invention relates to the effective cancellation of echoes in two-way communication circuits of extremely long length such as circuits completed by way of orbiting satellites or undersea submarine cables. It is well known that hybrid circuits containing two-wire to four-wire circuits do not provide echo free coupling between the receive and send lines of the four-wire circuit. A portion of the signal, typically voice signals on the receive line, will pass to the send line and appear as an echo signal. Because, in long line communications, the signals require a finite travel time, this reflected energy or echo is heard sometime after the speech is transmitted. As distances increase, in the case of satellite communications to tens of thousands of miles, the echo takes longer to reach the talker and becomes more annoying.
Voice operated echo suppressors are commonly used for removing the echo caused by the imperfection in the hybrid or other echo path. The suppressor operates to interrupt the send line whenever a voice level signal is detected on the receive line. This eliminates echo but, at the same time, eliminates some voice signals emanating from the local two-wire circuit resulting in a clipping of the outgoing conversation. The problems of echo suppressors in terms of clipping conversations has led to a new class of devices which substantially eliminate echo returns without impeding the free flow of conversation in both directions.
This new class of devices for handling echo problems are known as echo cancellers. An echo canceller does not interrupt the send line but generates an approximation, y(t), of the echo y(t) 41 ) and subtracts the former from the signal appearing on the send line. The remaining signal on the send line during double talk is, S(t) + e(t) where S(t) is the local voice signal and e(t) is the residual error caused by y(t) not being exactly equal to y(t).
In general, echo cancellers operate on the assumption that the echo path may be regarded as a filter wherein the relationship: ##EQU1## is satisfied; where f(t) is the signal applied to the echo path, k(.tau. ) is the impulse response of the echo path, and y(t) is the echo. Within the prior art, several solutions have been proposed which satisfy this relationship.
In Kelly et al, U.S. Pat. No. 3,500,000, a system is disclosed that automatically tracks variations in the echo path arising during conversation, for example, as additional circuits are connected or disconnected. The closed loop error control system of the latter patent includes an echo canceller which synthesizes a linear approximation of the echo transmission paths by means of a transversal filter. In a conventional manner, the filter comprises a delay line having a number of taps spaced along its length and develops a number of delayed replicas of the applied signal, each of which is independently adjusted in gain and polarity. The adjusted signals are then algebraically combined and then subtracted from signals in the outgoing circuit. The basic theory of operation and proof of convergence of the closed loop canceller as set forth in the Kelly et al patent is based on the linear treatment of a plurality of delayed signals, x.sub.i (t) adjusted in gain by a series of functions g.sub.i (t). Although the system achieves cancellation at workable levels, the structure is relatively costly to manufacture, primarily because of the tapped delay line, and is relatively large.
In Sondhi, U.S. Pat. No. 3,499,999, an improvement on the Kelly et al (U.S. Pat. No. 3,500,000) device is disclosed. The system described in Sondhi utilizes generalized filter networks in place of a tapped delay line system to obtain suitable convergence and effective suppression in a closed loop system. This patent is directed specifically to a closed loop echo canceller in which replicas of the echo signal reaching the return path of a two-wire to four-wire network junction are developed by passing incoming signals through a plurality of generalized filter networks to produce a number of linear transformations of the input signal. These transformations are then selectively adjusted in gain under control of a differential outgoing signal in a manner taught by the prior patent to Kelly et al.
In another implementation of adaptive echo cancellation devices known in the prior art, an X memory stores digitized samples of the incoming signal X(t) over a period T and an H-register stores a digital representation of the impulse response of the echo path. The H-register, a multi-stage register, containing the sampled form of the model of the echo path impulse response has its contents adaptively updated. The updating information for the H-register is obtained by implementation of an algorithm based on cross-correlating the receive side signal and the resulting echo signal from the echo path. In this form of implementation, both the X memory and the H-register recirculate, but the oldest sample in the X memory is replaced each sample period by a new sample of the signal X(t). Digital convolution is performed on the contents of the two memories, that is, the contents are multiplied sample-by-sample, and the products summed resulting in an approximation of y(t) of the echo.
In one case, the impulse response of the echo path is stored in the H memory by using the search or interrogating pulse technique. In this system, after the circuit is complete between caller and called stations, but before the conversation begins, an artificial search by interrogating pulse is applied to the receive line. The pulse passes through the echo path and the resultant signal on the send line is the impulse response of the echo path. The impulse response is sampled over a period T, digitized and stored in the H-register. This general technique is discussed in Flanagan et al, U.S. Pat. No. 3,535,473. For a number of reasons, however, including the fact that the impulse response of the echo path will not be constant, the search pulse technique is not satisfactory. More recent cancellers continuously compute an impulse response that minimizes the mean squared error between y(t) and y(t). The active circuitry includes an adaptive control loop, responsive to the residual error, e(t) and the receive side signal x(t) for implementing the steepest descent technique by adjusting the N samples of the H memory through incrementing or decrementing each sample by a given amount. When convergence is reached, i.e., the attainment of minimum error or echo, the contents of the H memory represent in digital form the impulse response of the echo path.
However, as is well known, when the echo signal is contaminated by other signals not originally present in the receive side of the echo canceller but, rather originating somewhere within the echo path, cross-correlation of such a contaminated signal results in erroneous information being stored in the H-register. The occurrence of contamination is unavoidable since it may consist of near-end speech which must be transmitted to the distant party. One method known within the prior art is to disable the updating process when the near-end speech is present. This near-end speech may represent contamination for the adaptive process. There is, however, a time constant involved in the disabling action which may, due to fast processing, allow erroneous information to be stored in the H-register. A second way that erroneous information enters the H-register is during the presence of high-level signals such as signalling tones, simultaneously present at receive and send sides of the canceller. Present cancellers remove the build-up and storage of erroneous information in the H-register after several seconds of receive speech and echo signals. The result, however, is the possibility of a spurt of audible echo for the distant talker.
The present invention prevents this from happening by zeroing all the contents of the H-register during one sampling interval upon detecting the erroneous information. The sampling interval is typically in the order of 125 microseconds. After the clearing action has occurred, the build-up of proper H-register information takes place within a fraction of a second and no noticeable echo occurs.